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Porpoising while towing fifth wheel

25K views 73 replies 24 participants last post by  hoppyz06 
#1 ·
I installed Air-Lift air bags and rear Bilstein HD shocks and now I have poropising of the 5er when riding over expansion joints and dips between the highway and bridges and some road expansion joints as well. I removed the Timbrens because I didnt like the unloaded ride but I never experienced porpoising with the Timbrens. I need to know what my next step is, do I go for something like a RoadMaster or Helwig springs? I saw something that dampens the existing spring at the shackles, will this work? I am trying to control the unloaded ride as well as the loaded ride without porpoising.

Thanks,
 
#2 ·
Have you considered a different pin box for the 5th wheel? They aren't cheap (unless you find one used) but can make all the difference in the world.
 
#3 · (Edited)
A porpoise is usually an overweight tongue or pin cause the springs don't have the backbone they need.

I would go the way of SuperSprings, Hellwigs, etc but they still have a pre-load so you will be stiffer un-loaded. That I'm afraid is a fact of life.

Follow me on this. Most Cedar Creeks are over 2K pins and that is the magic number on a 3/4 ton. Near or certainly beyond 2K the rear droops too much. Airbags could help but they are not really meant for lifting weight beyond what the stock springs can do. Many do it with satisfactory results but it isn't really the correct way to solve the problem. Airbags are meant to support or level a load not lift it.

The Pin boxes might do porpoising but they are really for chucking and yes the price is up there.
 
#4 ·
I'm wondering if you have too much air in your bags. My 5ers have had certain types of joints and bumps they porpoise on, and not others. I don't have any bags or special pinboxes (although it would be interesting at times to have a specialty pinbox.).

Cedar Creeks tend to be pretty heavy on the pin unless you load to the rear more.

You mentioned something interesting, and I'm wondering if this might be the factor - you said you didn't like the ride of the Timbrens unloaded. If I remember correctly, you shouldn't even really engage your Timbrens unless you have pretty good load. If you hitting them unloaded, you really must be highly inflated. Again, if this is not correct, I apologize.
 
#5 ·
I usually run the air bags at 45 psi when towing, I lowered it down to 25 psi and the porpoising diminished slightly. My Timbrens would normally sit approximately 1/2 inch off of the axle tube but when I would cross a speed bump or railroad track it would produce a violent snap back as the Timbren came in contact with the axle tube.
 
#6 ·
I run my bags at 30 lbs when towing. I found that when I ran 45 lbs in mine I would get porpoising. I would try and play around with the amount of air in the bags and see if you can find the sweet spot. Took me 4 trips to get mine where I like them.
 
#7 ·
All you need to do is adjust the air pressure in the air bags.

If I had to guess I would say you don't have enough air in the bags and are hitting the overload springs as the truck goes over the expansion joints (not enough air in the bags with a lot of pin weight) and then doing it again when the trailer goes over the joints-

Pump those bags up to 90 pounds and start working backwards in 10 pound increments until you start porpoising again at which point you know you went too low on air bag pressure.
 
#8 ·
I also feel it might be air in the bags related. Here are some simple steps to get to a very common base of starting and then fine tune the PSI.

- With no air in the bags and no extra load, take a measurement of your rear fender.

- Then add the air psi you use all the time and add the trailer/load. Take the rear fender measurement again.

What you are after is lets say your unloaded fender height is 45", you want to make sure you are not over 45" with air added in the bags and the trailer/load added to the truck. You want to have your suspension dropped into the springs about 1/2" +/-. Why? Because if you are over 45" then you are unloading the rear leaf springs and now using the bags more than assisting the leaf springs and you can see a bad ride reflection as in porpoising or the rear end tracking funny.

If you are sunk into the suspension alot with the air bags you can also feel porpoising but the rear will not feel as loose.

I would start by looking at the PSI and making sure you are within a good starting spot and after that depending on what you find I can suggest a couple more things to check.

Also I might add the reason you didn't feel this with the Timbrens was they are just a large bump stop and if the suspension can't move ...well there can not be porpoising.

Also, are you on stock size tires and not cranked up on the fron end? No rear added lift? Don't think you are but just making sure we are not over looking anything.

Keep us posted on what you find...
 
#21 · (Edited)
Nick,

Thank you for all of the good advice, I have taken some measurements and posted them in an excel file below. I think that you are on the right track with excess air in the bags. I took particular note of the distance between the spring pack and the overload spring. With excess air in the bags, it appears that I am supporting the load off of the overload springs and when I hit a bump, I am assuming that the overload spring kicks in hard. It took 75 psi in the bags to get any clearance on the rear overload spring pad To me, it seems that 10 psi (minimum recommended pressure) towing is what I want to try next, any comments will be appreciated.
 

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#9 ·
Thank you for all of your great advice. The truck and tires are bone stock with no cranking of the front end. I am starting to see that the problem is really coming from the excess air pressure. I did notice an improvement when I went from 45 psi to 25 psi. I will measure the static height and adjust from there. When I originally aired up the bags in excess of 45 psi, the trailer started to tail dive and the clearance between the bed rails and the RV was way too close for comfort. It sounds like I have some work to do but I will get it right with your help.

Has anyone had any luck with the Roadmaster Active Suspension product? It looks very interesting.

Thanks for all of your help
 
#10 ·
You might want to check your tire pressure too. When we got our trailer, I assumed I should run the rear tires at 80 p.s.i., but doing so wore the center of the tires out. Turns out that even with the trailer pin weight, I am 850 pounds under the axle maximum and over 1,000 pounds under the tire ratings.
 
#11 ·
You might want to check your tire pressure too. When we got our trailer, I assumed I should run the rear tires at 80 p.s.i., but doing so wore the center of the tires out. Turns out that even with the trailer pin weight, I am 850 pounds under the axle maximum and over 1,000 pounds under the tire ratings.
I do run my tire pressures between 75 to 80 lbs in the rear. I know that I am close to or slightly over my weight ratings on the rear wheels. I dont feel safe running lower air pressure. I also have a TPMS on the trailer and truck tires just to avoid any low pressure or high heat situations.
 
#12 ·
http://s219.photobucket.com/albums/cc64/racechaser1/air bags/?
I have posted pictures of the air bag installation if anyone can spot any irregularities. The pictures are taken with 10 psi in the bags and nothing in the bed except the super glide hitch which weighs around 200 lbs. I installed a tee in the air lines so both bags are equal pressures.
Thanks again
 
#16 ·
I think the jury may still be out on the "T" idea. Hitting a bump with one side would cause it to compress and send the pressure to the other side. Running two lines would make the pressure effect only the side hitting the bump, which is how the bag is supposed to operate - I think!? :confused:
 
#13 ·
I am still playing with my air bags. So far I have come up with this. I have 6000 lbs on the rear axle and I run 35 psi in the bags. This gives me about 1/4 inch off the overloads. I still get a bit of porpoising, but I do hit a nasty section of road there too. I am going to try to bump the psi up another 5 on the next trip. I run level with 35psi in the bags and have cranked up the T-bars.
 
#56 ·
#14 ·
I usually run 80 to 110 psi in my bags with a 4000 lb pin weight an the ride is pretty smooth. a cushioned pin box or fith wheel hitch will make alot of difference if you think you are loaded heavy.
 
#15 ·
I run 90 pounds when I have toys in my trailer (less pin weight) and 100 pounds without toys (more pin weight) in my air bags but then my trailer is probably heavier than most- I get no porpoising and a great ride.

It is my belief (and I could certainly be wrong) that most porpoising is caused by the truck bouncing on and off the overload springs-

In a situation where your rear suspension loads enough to engage the overload springs you want to control the rebound with good shocks and have enough air in the bags to keep the suspension from cycling through another loading and un-loading of an event that is now 200 or more feet behind you.

Now I am no truck suspension guru but I know a little about adjusting suspension on motorcycles- If you pump your bags up enough you are in effect increasing your pre-load and your rebound. This will do two things- It will keep the back of your truck out of the overload springs more often but also when you do engage them the stroke won't be as deep and the truck will return to rest (normal position) sooner.

The great thing about the bags is you can play around with them until you get things just right- but I would start with more and not less air.
 
#20 ·
.....It is my belief (and I could certainly be wrong) that most porpoising is caused by the truck bouncing on and off the overload springs-

In a situation where your rear suspension loads enough to engage the overload springs you want to control the rebound with good shocks and have enough air in the bags to keep the suspension from cycling through another loading and un-loading of an event that is now 200 or more feet behind you.

Now I am no truck suspension guru but I know a little about adjusting suspension on motorcycles- If you pump your bags up enough you are in effect increasing your pre-load and your rebound. This will do two things- It will keep the back of your truck out of the overload springs more often but also when you do engage them the stroke won't be as deep and the truck will return to rest (normal position) sooner.

The great thing about the bags is you can play around with them until you get things just right- but I would start with more and not less air.
x2. I would also get rid of the T-fitting. If your worried about drilling a second hole, I put my airline valves on the same plate the 7 way connector is hanging in. Put some black caps on them and no one will even notice them.
 
#18 ·
My 2002 has the two-stage rear springs. First stage is the multiple pack, and the second stage is the single thick spring.
I tow a 12K 5ver with pin weight of 2-2.5k lbs.
If I air up >25 PSI, the first stage starts to disengage with second stage and I notice a little sway.
I usually run 18-20 PSI in bags which levels truck, keeps all springs engaged, and rides very well. Rear tires at 80 PSI when towing.
 
#19 ·
I'd take that T out of the line and run two separate lines. That should help stop any side to side rocking.
 
#27 ·
Thanks again for all of the advice. I have been towing at 45 PSI and it was the worst porpoising experienced. At 25, it was better. The trailer definitely became nose high at 45 psi, At 25 psi it was slightly nose high but not too bad. I am afraid to lower my pin box height as the clearance between the bed rails and the 5er will become too tight. I am also afraid to raise my 5er axle height on the trailer because I am already 13 ft 3 inches on height and I just make it below some of the bridges in my area. The superglide hitch has no height adjustments so I am at a loss on how to lower the pin height. My thought was to lower the psi down to 10 in the bags which will take out the possibility of lifting the suspension too much and also lower my nose height on the 5er. Other than cranking the torsion bars on the truck to possibly lower the rear or adding Roadmaster Active Suspension, I am at a loss.
 
#25 ·
Also, when you add air bags and height, make sure you drop your pin box on your trailer 1 notch to get the weight distribution back onto your truck. I had similar issues when first using air bags back in '92 and that cured the problems you describe. Don't lower the hitch height if you can, but drop the pin box.
 
#26 ·
WoW!! I just purchased the Air Lift 5000s today and I had no idea what I was getting myself into!

Thanks for all of the information you guys are sharing with us newbies!!

The system will be installed next week in time for a 2000 mile trip the following week!! Hopefully by the time I get back I will have all of the bugs worked out. BUT at least now I have a place to start on how to adjust these things. Thank goodness I'm ordering the AirLeft Remote system so I can make all of these adjustments on the fly and from the driver's seat!! Also glad to have the presets as I sometimes tow with and without toys which should make a huge difference in pressures!!

Keep up the conversation so we can all keep learning!!

Thanks,

Nicholas Traub
 
#30 ·
If the noise is up that high you are not putting the weight over each axle even. I also had my duallys back end porpoising when it was noise high. I think you have to at least know that the trailer vs. truck stance is correct before you can pin point the exact problem because it all add's up for the best results. You might be able to hide a problem with a mod but not sure myself how to get it to act better with out strutting out the rear end to be stiff and that's not the answer, if it was we would all have to be doing that. I feel the trailer noise needs to come down, going off what I found on my trailer and what I changed. Everything else is about the same of how we are set up.
 
#31 ·
if you could post a pic of your tow rig hooked to your 5er it may help get you a more definite answer. i dont know how much clearance you need between your trailer and the rails on your truck but i have 4 inches between mine and i have never got close to hitting them.
 
#32 ·
Land vehicle Vehicle Transport RV Car


Land vehicle Vehicle Car Transport RV


Here are some pics of the setup with the Timbrens installed. I was always a little nose high but the Timbrens produced no porpoising. I will be pretty close to this height when I lower the air bag psi. I have hit the bed rails with the 5er slightly on several occasions when going out of driveways etc. so I am afraid to get closer to my bed rails by lowering the pin box.
 
#33 ·
off topic, but lovely looking rig and trailer combo....

-Rob
 
#34 ·
looks like you have your ride height just about right. id try what norcal nick suggests an see how that works
 
#35 ·
Ok...so here is a really simple question!!

With all of the pressure recommendations, are those pressure suggestions with load or without load. I'm assuming that will make a difference?!?! A little clarification would be great before I start this same process next week!!

Thanks!!
 
#36 ·
air pressure should be checked LOADED.
 
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